WO2008019670A2 - Method for generating oxidic nanoparticles from a material forming oxide particles by film evaporation - Google Patents

Abstract

The invention relates to a method for generating oxidic nanoparticles from a material forming oxide particles, comprising the steps: preparation of an aqueous solution containing the ions of the material forming the oxide particles, film evaporation of the solution at a temperature above 200 °C and skimming off the nanoparticles floating on the surface of the aqueous solution generated in the vicinity of the vapour film on film evaporation.

Description

 A method of producing oxide nanoparticles from an oxide particle-forming material

The invention relates to the production of oxide nanoparticles from an oxide particle-forming material, in particular metal oxide particles, and to the formation of a film of such particles on any substrate.

Nanostructures, which themselves are nanoparticles or formed by even distribution and / or regular arrangement of nanoparticles on a substrate, are currently the focus of current research. They represent a class of materials that u. a. due to quantum effects have novel electrical, optical, magnetic and thermodynamic properties. Alongside the primarily academic questions, there is also the problem of reproducible mass production with the simplest possible means for promptly reaching the area of commercial use of nanostructures.

For the general state of the art relating to the production of nanostructures, reference is made to the following references: D. Rovillain et al .: "Boiling Chemical Vapor Infiltration - An Experimental Study on Carbon / Carbon Composites", Carbon 39 Urban and CT, Avedisian, W. Tsang: "The Film Bioling Reactor: A New Environment for Chemical Processing", pp. 1355-1365 (2001);

AlChE J. 52 (7), pp. 2582-2595 (2006); DE 10 2005 060 407 B3; DE 103 92 447 T5; Dongsheng Wen and Yulong Ding: "Experimental investigation into the pool boiling heat transfer of aqueous γ-alumina nanofluids", Journal of Nanoparticle Research 7, pp. 265-274 (2005) and SM You, JH Kim, KH Kim: "Effect of nanotechnology particles on critical heat flux of water in pool boiling heat transfer ", Appl.

Lett. 83 (16), pp. 2274-2276 (2003). In particular, the production of a uniformly distributed film of nanoparticles on the surface of a substrate is still a difficult process, which is usually associated with several steps and high costs, especially if the nanoparticles are to be arranged in a certain way. Typical processes for the preparation of such structures are Vapor Liquid Solid (VLS) or MOCV processes. Although these methods are relatively universally applicable, but both the control of the atmosphere (UHV) and the need for high temperatures (600-1000 ⁰ C) require expensive equipment and make the synthesis time-consuming. Pre-structured substrates such as MEMS can not readily be exposed to such high temperatures.

To reduce the expense, the person skilled in the art also knows aqueous-solution wet-chemical preparation processes which achieve the desired results at temperatures below 100 ° C. and at atmospheric pressure (see, for example, Law et al.

However, the wet-chemical methods have other disadvantages in addition to their very slow process (process times of several hours to days), such as no epitaxial growth on silicon possible (see J. Phys.Chem.A B 2001, 105, 3350-3352) In addition, solvents are also sometimes used which cause a disposal problem.

For example, one is currently very interested in the production of zinc oxide (ZnO) nanostructures, such as nanorods and nanotubes. This is the case because ZnO, as a semiconductor, can form a large variety of nanostructures.

Moreover, versatile applications as optoelectronic devices, lasers, field emission and gas sensor materials are considered (for the fabrication and application of nanotubes and nanorods see also Advanced Materials 2005, 17, 2477). In order to epitaxially produce ZnO structures, either special substrates such as gallium nitride (GaN) are used or silicon substrates are coated with a seeding layer, which usually consists of a ZnO thin film heated at 400 ° C.

A direct, not eptitaktische production of distributed ZnO structures on substrates is shown in the patent application DE 10 2005 060 407 Al. In this case, metal salts, in particular zinc acetate, are dissolved in water and applied to a previously heated substrate (eg heating plate) dripped. The droplets evaporate, floating on the vapor cushion (Leidenfrost effect), and on the wetted substrate surface remain distributed nanostructures, especially about small tufts of ZnO wires.

The Leidenfrost effect appears essential for the formation of nanoparticles from the metal ions previously dissolved in water. For this purpose, a very fast reaction kinetics in the non-equilibrium at the phase boundary fluid / water vapor at the bottom of the drop is taken as the cause. The exact nature of the processes there is not yet clear.

However, in addition to further advantages, for example with respect to the directional arrangement of nanowires, the method described in DE 10 2005 060 407 A1 has the disadvantage that it is not readily suitable for coating any desired substrates. Obviously, there are problems with curved, locally recessed or even angled substrate surfaces, if the vaporizing drops are to slide over it. Moreover, the substrate must withstand temperatures above 200 ^° C. for a longer time - although it should not oxidize, which restricts the selection of materials, inter alia, in the field of plastics.

However, it has been noted in a recent work (Urban and Avedisian, "The Film Boiling Reactor: A New Environment for Chemical Processing," AIChE Journal, 52, 1, pp. 2582-2595 (2006)) that the process Film boiling, similar to the Leidenfrost effect, enables effective chemical reactions. Specifically, a so-called FIBOR ("film boiling reactor") for the production of hydrogen gas from methanol is described there.

It is therefore the object of the invention to provide a method for the rapid production of nanoparticles, which at the same time allows a very simple application of films of such particles aufweitgehend any substrates.

The object is achieved by a method having the features of the main claim. The dependent claims indicate advantageous embodiments of the method.

According to the invention, the generation of the nanoparticles by chemical reaction of ions dissolved in water, in particular metal ions, at the phase boundary of water. - A -

water vapor is provided between the water body and a body of water heat supplying surface (e.g., hot plate). In this respect, the concept of particle production is taken from DE 10 2005 060 407 A1. However, the realization of this configuration does not occur here by dripping the solution, but by film boiling in an optionally continuously operating reactor. The acronym

FIBOR is assumed below for this reactor.

The invention further provides that at least the top of the body of water is freely accessible. It turns out, namely, that the nanoparticles formed according to the invention undergo buoyancy after their formation, which preferably takes place on the underside of the water body, and finally float in large numbers on the water surface. Upon immersion of a substantially arbitrary substrate into the body of water from the surface provided with floating particles they bind immediately to the substrate, so that the substrate is coated after pulling out with a relatively firmly adhering nanoparticle film.

The invention will be explained in more detail below. The following illustrations also serve this purpose:

FIG. 1 outlines the different boiling behavior of water for different values of the temperature difference ΔT = Twan _d - _Twereer between a wall emitting heat energy and the water body itself contacting the water;

Fig. 2 shows the known "boiling curve" for water, in which the amount of heat transferred into the body of water is plotted against ΔT;

3 shows a silicon substrate coated according to the invention with ZnO particles.

A FEBOR realizes in a manner known per se the continued film boiling of a liquid. Especially for water, it is known that the heating of the water body differs from one of its interfaces as a function of the temperature difference ΔT between body of water and heating surface. For ΔT <5 K, the heat transfer is predominantly convective and with sporadic formation of isolated water vapor bubbles on the heated wall, which possibly detach, rise in the water body and thereby dissolved again (see Fig. 1 left picture). If ΔT is further increased to values of up to 30 K, increased bubble formation takes place gradually, whereby these are formed over a large area, in a more rapid manner

Remove the string and ascend to the water surface (see Fig. 1 middle picture). From Fig. 2 it can be seen that the amount of heat dissipated in the body of water from the beginning of the increased bubbling until reaching ΔT = 30 K increases particularly strong.

In fact, ordinary household stoves are designed for this reason so that the stoves reach absolute temperatures around 130 ⁰ C.

FIG. 2 also shows the finding that the heat transfer into the water body decreases again for even higher ΔT and reaches a minimum at approximately ΔT = 200 K. The reason lies in the increased evaporation in the immediate vicinity of the heated wall. The steam can no longer be absorbed and removed by the body of water quickly, so that an insulating cushion of steam between the water and the wall forms over ever larger areas. Herein lies the analogy to the known Leidenfrost effect. The minimal heat transfer apparently occurs when the entire heating surface is covered by a vapor film, which largely prevents the convective heat transfer. This state marks the onset of film boiling (see Fig. 1 right picture). A FlBOR has the task of inducing film boiling and maintaining it by means of suitable control, such as heating temperature.

The exact design of the FIBOR lies within the scope of the expert's actions. However, it should be emphasized that a FIBOR advantageously designed for the purpose of the invention should allow free access to the surface of the water body in order to carry out the inventive dip coating of substrates with nanoparticles. Therefore, a FIBOR appears as an open trough a heater acting from the underside is configured, particularly suitable for the realization of the invention.

The following general advantages of the invention are to be emphasized: 1. The erfmdungsgemäße method requires no defined Ataiosphäreibedin- conditions, and in particular no pressure is required. It can be done in indoor air in any laboratory.

2. The FIBOR only needs to be supplied with energy (heat) and an aqueous metal salt solution that can be produced quickly and at short notice. Catalysts are not absolutely necessary, and in principle no dangerous by-products are formed.

3. The generation of nanoparticles from the aqueous solution, the Auftreiben the

Nanoparticles on the water surface and the coating of a substrate by immersion in the solution require a total of only a few minutes. The process is thus many times faster than other wet-chemical processes in which particle growth takes place.

The simplest, at any time to realize design of a suitable FEBOR is a simple beaker on a hot plate. Such a "beaker reactor" will not be able to sustain continuous production of nanoparticles without further action, but these additional measures are considered to be expertly practicable, as stated above.

FIG. 3 shows electron microscope images of a film of zinc oxide nanoparticles prepared according to the invention on a silicon substrate in two magnifications. To make the film a laboratory hotplate was initially preheated to 250- 270 ⁰ C. A 100 ml beaker of aqueous zinc acetate solution was placed on the preheated plate. Measurements prove that the temperature of the underside of the glass corresponded to that of the heating plate after 1-2 minutes.

After about 10-15 minutes, a film floating on the water surface formed, and a silicon substrate was slowly and uniformly dipped in the solution over the period of about one minute and pulled out again. The previously floating film remained adhered to the substrate during the dipping motion.

The images from Fig. 3 prove that it is relatively evenly distributed, about the same size nanoparticles. Their composition was determined by means of X-ray diffraction (XRD) confirmed as ZnO particles by X-ray analysis (EDX) and X-ray diffraction (X-Ray diffraction).

Repeating the immersion process with a second silicon substrate after a few minutes forms a nanoparticle film whose particle size distribution spreads further than that of the film on the first substrate. The reason for this is, on the one hand, the agglomeration of the already existing nanoparticles and, on the other hand, the addition of further ions from the solution to these particles, i. Particle growth progresses over time. This fact must be taken into account in the implementation of the procedure in a continuous FIBOR.

Since the substrate is coated with the particle film by immersion in water under atmospheric pressure, the temperature to which the substrate is exposed is practically limited to about 100 ^° C. upward. Therefore, both prestructured and some organic substrates can be provided for coating, provided that only the particles produced adhere to them. Of particular interest here are substrates of commercially available, chemically inert polymers such as polymethymethacrylate (PMMA) or any fluoropolymer (eg Teflon®). The adhesion of the nanoparticles can be favored by a controlled, temperature-induced softening of the polymers.

The example zinc oxide should not be understood as limiting in analogy to DE 10 2005 060 407 A1. Research in this field is still in its infancy, but it is already clear today that other particles, in particular metal oxide particles, can also be produced in the same way. Of particular commercial importance is titanium dioxide (e.g., as a UV absorber).

The oxygen to form the oxide particles may be provided by thermal dissociation of water molecules. However, it is also conceivable (and not yet clarified) that the oxygen dissolved in the solution plays a role in the reaction. If this is confirmed, the monitoring and, if necessary, control of the dissolved oxygen concentration should be considered when constructing a continuous FIBOR. This can be realized by a simple gas pump, which sucks in, for example, room air and via a pipe in the reactor vessel lent possible in the vicinity of the reactive water / water vapor phase boundary passes. Alternatively, one can also initiate pure oxygen from bottles. Finally, it should be noted that the film boiling is a fixed term, which is associated with the minimum of heat transfer into the fluid as a starting point. In the present invention, however, the term film boiling should also refer to the case of a possibly (not yet) contiguous vapor film (usually referred to as "transition boiling"), insofar as it is already capable of producing nanoparticles 10 2005 060 407 Al it can be assumed that the nanoparticles can be formed as early as above a wall temperature above 200 ° C, where classic film boiling does not necessarily have to be used, which is why film boiling in the sense of the invention should be understood broadly.

Claims

claims 1. A method of producing oxide nanoparticles from an oxide particle-forming material, comprising the steps of: Providing an aqueous solution containing ions of the oxide particle-forming material, - Film boiling the solution at temperatures above 200 ⁰ C, and Skimming the nano-particles produced in the region of a vapor film during film boiling and floating on the surface of the aqueous solution. 2. A method for coating a substrate with nanoparticles produced by the method of claim 1, characterized in that the surfaces of the substrate to be coated are immersed in the aqueous solution in such a way that they traverse the nanoparticles floated to the surface of the solution. 3. The method according to claim 2, characterized in that the substrate is immersed in the solution at a substantially constant speed and pulled out again. 4. The method according to any one of the preceding claims, characterized in that as the oxide particles forming material, a metal is selected. 5. The method according to claim 4, characterized in that the oxide particles forming material is selected from the elements zinc or titanium. 6. The method according to any one of claims 2 or 3, characterized in that a substrate containing at least one organic material is used. 7. The method according to claim 6, characterized in that the organic material is polymethylmethacrylate (PMMA) or a fluoropolymer. 8. The method according to any one of the preceding claims, characterized in that the aqueous solution with ions of the oxide particle-forming material oxygen or room air is supplied in controllable amount in the region of the vapor film. 9. Device for carrying out according to one of claims 1 to 7, characterized by an upwardly open trough for receiving the aqueous solution with ions of the oxide particle-forming material, a heater at the bottom of the trough, - A control device for maintaining a non-contiguous vapor film comprising a thermometer for controlling the heating power of the heater and - An adjustable device for gas supply in the area of the trough bottom.